1. Field of the Invention
This invention pertains to microminiature thermionic converters having high energy-conversion efficiencies and variable operating temperatures, and to methods of manufacturing those converters using semiconductor integrated circuit fabrication and micromachine manufacturing techniques. The microminiature thermionic converters (MTCs) of the invention incorporate cathode to anode spacing of about 10 microns or less and use cathode and anode materials having work functions ranging from about 1 eV to about 3 eV.
2. Description of the Related Art
Thermionic conversion has been studied since the late nineteenth century, but practical devices were not demonstrated until the mid-twentieth century. Thomas Edison first studied thermionic emission in 1883 but its use for conversion of heat to electricity was not proposed until 1915 by Schicter. Although analytical work on thermionic converters continued during the 1920's, experimental converters were not reported until 1941. The Russians, Gurtovy and Kovalenko, published data which demonstrated the use of a cesium vapor diode to convert heat into electrical energy. Practical thermionic conversion was demonstrated in 1957 by Herqvist in which efficiencies of 5-10% were reached with power densities of 3-10W/cm.sup.2.
FIG. 1 illustrates the components and processes of a typical thermionic converter employing technology understood and applied prior to the present invention. A heat source 15 elevates the temperature of the emitter electrode 10 (typically, between 1400-2200 K). Electrons 50 are then thermally evaporated into the space, or interelectrode gap (IEG) 5, between the emitter electrode 10 and collector electrode 20. The electrodes are operated in a vacuum, near vacuum, or in low pressure vapor (less than several torr) 65 within a vacuum or rarefied vapor enclosure 60. The collector electrode 20 is cooled by a heat sink 25 and kept at a low temperature. The electrons 50 travel across the IEG 5 toward the collector electrode 20 and condense on the collector electrode 20. The electrons 50 then return to the emitter electrode 10 through the electrical leads 30, electrical terminals 35 and load 40 which connect the collector to the emitter. The figure shows an example configuration wherein the rarefied enclosure 60, itself, functions as a conduit of heat addition on one side and heat removal on the other. Alternatively, it is possible for the heat source and heat sink to be positioned inside enclosure 60 and function independently from it.
Thermionic emission depends on emission of electrons from a hot surface. Valence electrons at room temperature within a metal are free to move within the atomic lattice but very few can escape from the metal surface. The electrons are prevented from escaping by the electrostatic image force between the electron and the metal surface. The heat from the emitting surface gives the electrons sufficient energy to overcome the electrostatic image force. The energy required to leave the metal surface is referred to as the material work function, .o slashed.. The rate at which electrons leave the metal surface is given by the Richardson-Dushman equation: EQU J=AT.sup.2 exp(-e.o slashed./kT),
where A is a universal constant, T is the emitter temperature, k is the Boltzmann constant, and .o slashed. is the emitter work function. Large emission current densities are achieved by choosing an emitter with low work function and operating that emitter at as high a temperature as possible, with the following limitations. Very high temperature operation may cause any material to evaporate rapidly and limit emitter lifetime. Low work function materials can have relatively high evaporation rates and must be operated at lower temperatures. Materials with low evaporation rates usually have high work functions.
Choosing the correct electrode material is a key component of designing functional thermionic converters. A general description of suitable materials is presented here in association with disclosing the principles of the converters of the present invention. Example materials suitable for the microminiature thermionic converters of the present invention and others (as well as methods for making them) are disclosed in a separate patent application Ser. No. 09/257,336 filed on the same day as the present application. That separate patent application is incorporated herein in its entirety.
Once the electrons are successfully emitted, their continued travel to the collector must be ensured. Electrons that are emitted from the emitter produce a space charge in the IEG. For large currents, the buildup of charge will act to repel further emission of electrons and limit the efficiency of the converter. Two options have been considered to limit space charge effects in the IEG: thermionic converters with small interelectrode gap spacing (the close-spaced vacuum converter) and thermionic converters filled with ionized gas.
Thermionic converters with gas in the IEG are designed to operate with ionized species of the gas. Cesium vapor is the gas most commonly used. Cesium has a dual role in thermionic converters: 1) space charge neutralization and 2) electrode work function modification. In the latter case, cesium atoms adsorb onto the emitter and collector surfaces. The adsorption of the atoms onto the electrode surfaces results in a decrease of the emitter and collector work functions, allowing greater electron emission from the hot emitter. Space charge neutralization occurs via two mechanisms: 1) surface ionization and 2) volumetric ionization. Surface ionization occurs when a cesium atom comes into contact with the emitter. Volumetric ionization occurs when an emitted electron inelastically collides with a Cs atom in the IEG. The work function and space charge reduction increase the converter power output. However, at the cesium pressures necessary to substantially affect the electrode work functions, an excessive amount of collisions (more than that needed for ionizations) occurs between the emitted electrons and cesium atoms, resulting in a loss of conversion efficiency. Therefore, the cesium vapor pressure must be controlled so that the work function reduction and space charge reduction effects outweigh the electron-cesium collision effect. An example of an operational thermionic converter is that found on the Russian TOPAZ-II space reactor. These converters operate at the emitter temperatures of 1700 K and collector temperatures of 600 K with cesium pressure in the IEG of just under one torr. Typical current densities achieved are&lt;4 amps/cm.sup.2 at output voltages of approximately 0.5 V. These converters operate at an efficiency of approximately 6%. The control of cesium pressure in the IEG is critical to operating these thermionic converters at their optimum efficiency.
A variety of thermionic converters are disclosed in the literature, including close-spaced converters. (See: Y. V. Nikolaev, et al., "Close-Spaced Thermionic Converters for Power Systems", Proceedings Thermionic Energy Conversion Specialists Conference (1993); G. O. Fitzpatrick, et al., "Demonstration of CloseSpaced Thermionic Converters", 28.sup.th Intersociety Energy Conversion Engineering Conference (1993); Kucherov, R. Ya., et al., "Closed Space Thermionic Converter with Isothermic Electrodes", 29.sup.th Intersociety Energy Conversion Engineering Conference (1994); and G. Oi. Fitzpatrick, et al., "Close-Spaced Thermionic Converters with Active Spacing Control and Heat-Pipe Isothermal Emitters", 31.sup.th Intersociety Energy Conversion Engineering Conference (1996).) Previously demonstrated thermionic converters, however, have not been able to achieve the current densities and conversion efficiencies predicted for the present invention. Others' efforts in the field of close-space converters demonstrate that expense and difficulty arise as a result of separately manufacturing and assembling at close tolerances the converter components such as the emitter, collector and spacers. additionally, the assembly process results in relatively large converters with spacing between the emitter and collector of up to several millimeters. A large gap spacing between the emitter and collector causes the energy conversion efficiency to drop dramatically, often necessitating Cs vapor systems even in converters otherwise designed to be "close-spaced." Such vapor systems are usually large and cumbersome, and precise control of Cs vapor pressures needed to maximize conversion efficiency (ensuring that space-charge reduction effects outweigh electron-Cs collision effect) is difficult.
Miniature thermionic converters without ionized positive vapor in the IEG offer the simplest solution to thermionic energy conversion. The small IEG size itself reduces the density of electrons in the gap (and their resulting current limiting space charge). As alluded to above, the close-spaced converter has historically been difficult to manufacture for large-scale operation due to the close tolerances (several microns or even submicron interelectrode gap size) needed for efficient operation. As demonstrated below, however, large scale production and operation of these close-spaced converters is now possible using IC fabrication techniques according to the principles of the present invention. Spacings on the order of 0.25 microns can now be produced and maintained over relatively large emission areas. Also, the development of low work function electrodes eliminates the need for gas adsorption to lower the electrode work functions.
The MTC has application both in government and in industry. MTCs could be retrofitted into almost any system requiring energy conversion from heat to electricity. MTCs are suitable for use in satellite and deep space missions where conventional thermionics alone and in conjunction with radioisotope thermal generators are currently used or planned. Increasing the efficiency of current fossil fuel plants and systems as well as introducing new technologies for increasing the efficiency an utility of renewable energy supplies such as solar would help to reduce U.S. dependency on fossil fuel consumption. Combustion heated MTCs could be used for high efficiency conversion of heat to electricity as stand alone units or as part of topping cycle or bottoming cycle cogeneration systems in larger central power plants. They are also suited to use in the new smaller gas fired combined-cycle plants that utilities are building to meet peak power demands. At lower power scales (typically less than 125 kWe), MTCs could prove to be more economical than conventional cogeneration systems using machinery with moving parts. Smaller mechanical systems have shown increased operating costs due to increased maintenance requirements. Very small MTC units (1-50 kWe) could be used with home heating systems (furnaces and water heaters) and small businesses to feed electricity back into the home/business or its community electric grid. MTCs could also be used with solar concentrators or central receiver power towers to generate electricity as stand alone units or in conjunction with other conversion technologies. These applications could by linked to an existing power grid or be deployed in any undeveloped region without a grid (eliminating the need in those areas for developing an expensive electric power grid).